Traditionally, hops, in the form of hop cones or hop pellets, are added during the wort boiling stage of the brewing process. This results in the extraction of alpha-acids (humulones, such as humulone, adhumulone, cohumulone, posthumulone and prehumulone) into the sweet wort, which under the influence of temperature (thermal isomerisation) are partially isomerised to the corresponding iso-alpha-acids (isohumulones). These iso-alpha-acids are responsible for the characteristic bitter taste of hopped beer. Typical alpha-acid levels in the wort at the start of the wort boiling are below 0.01 wt % (below 100 ppm). Disadvantages of this traditional hopping approach to impart bitterness are the inefficient alpha-acid extraction and isomerisation at the wort pH, i.e. a pH between 5.2 and 5.4, typically resulting in iso-alpha-acid yields below 40% (GB 1,158,697).
It became clear that the hop utilization could be improved by performing the alpha-acid isomerisation outside the brewing process, and particularly under the effect of basic inorganic compounds. One patent uses powdered hops as process feed which are mixed with alkaline earth metal oxides and subsequently heated under anaerobic conditions (U.S. Pat. No. 4,123,561). Disadvantages of this process are off-flavour formation and low hop utilization. Therefore, the later developments used hop extracts, obtained by extraction of hop resins (among others the alpha-acids) from the hop cones, rather than powdered hops.
The use of hop extracts instead of the traditional hop products has multiple advantages, among which a more stable and more consistent chemical composition. Generally, such hop extracts are nowadays obtained via liquid or supercritical carbon dioxide extraction, which eliminates the problems of the presence of pesticide and solvent residues in the extracts experienced before when applying organic extraction solvents like hexane. Carbon dioxide hop extracts provide predominantly alpha-acids (humulones) next to beta-acids (lupulones), and they can be further fractionated to obtain alpha-acid enriched hop extracts.
Starting from these alpha-acid containing extracts, the “off-line” pre-isomerisation of alpha-acids outside the brewing (wort boiling) process can be achieved using alkali metal and/or alkaline earth metal based compounds via two approaches. The first approach applies a solvent-free reaction medium while in the other approach the transformation is performed after addition of solvents, either pure water or alternatively water mixed with an organic solvent.
The first approach is generally based on alkaline earth metal compounds, for example alkaline earth metal oxides (e.g. MgO). These inorganic accelerators are mixed with the alpha-acid containing extract. However, during these isomerisation processes, the oxides are at least partially dissolved and therefore, after a long reaction time at elevated temperature, the iso-alpha-acid products are obtained as alkaline earth metal cation-isohumulate complexes (U.S. Pat. No. 5,015,491) with an isomer distribution of cis- and trans-iso-alpha-acids of 50:50. To obtain the iso-alpha-acids as an organic phase, an additional step is required which involves acidification (with e.g. an aqueous solution of sulphuric acid) of the alkaline earth metal isohumulate salts. This leads to an iso-alpha-acid layer, which can be decanted from the separate aqueous layer containing the alkaline earth metal sulphate salts. To obtain the commercial product, a neutralisation process with a potassium compound is required which results in an aqueous solution of dissolved potassium isohumulates. This process type implies that the initially added alkaline earth metal compounds cannot be recovered as such from the products.
In the other approach, alkali metal salts and/or alkaline earth metal salts are used, e.g. K2CO3 and MgCl2 solutions (U.S. Pat. Nos. 3,765,903; 3,952,061; 4,002,683; 4,758,445). Stoichiometric quantities of alkali metal cations and high temperatures are generally required to convert the alpha-acids into iso-alpha-acids in the water containing reaction medium. Again, to isolate the iso-alpha-acids, an acidification and a decantation step are required. In addition, the presence of water leads to the formation of unwanted degradation by-products, e.g. humulinic acids.
In U.S. Pat. No. 5,370,897, an alkaline earth metal salt (e.g. MgSO4) and an alkali metal salt (K2CO3) were combined into one process to speed up the alpha-acid isomerisation.
U.S. Pat. No. 5,155,276 describes the application of a heterogeneous accelerator viz. Al2O3, additionally requiring a dissolved alkali metal salt in at least stoichiometric molar ratio to the alpha-acid reactant. Although in this process an ‘insoluble catalyst’ is used which can be separated from the reaction mixture by filtration after the isomerisation reaction, the heterogeneous accelerator is by itself not effective, and needs a second, dissolved accelerator. Moreover, additional acidification and decantation steps are required to obtain the iso-alpha-acid product.
The above clearly shows that the isomerisation processes of hop alpha-acids known in the art are complex operations, comprising the use of soluble reaction accelerating inorganic compounds and including acidification and decantation to remove the inorganic accelerators, thus resulting in high levels of waste products, with often the unwanted formation of degradation by-products. Thus, there remains a need for improved methods for the isomerisation of hop alpha-acids.